Radio frequency powered sealing apparatus has been proposed heretofore for heat sealing a single set of superimposed strips of plastic sheeting together including means for forming a tear seal contiguous to the heat seal to facilitate separation of a finished item from stock material. A great variety of finished articles of manufacture have been sealed together utilising this heat sealing technique but the equipment and methods employed heretofore have been restricted in various respects. For example, only a single item is commonly processed by prior equipment including means for indexing strip material past a heat sealing station. The U.S. Pat. No. 2,758,631 granted to Peterson et al on Apr. 14, 1956 discloses apparatus having these features. Techniques have also been proposed for forming plastic cushion covers using heat sealing equipment in which the bottom and top plastic covers are heat sealed to the lateral edges of a gusset strip. The gusset strip is formed into a ring of appropriate size and this is assembled along the interior of a sealing die ring, the edges of the gusset being overturned and suitably anchored in place along the exterior of this die. This complex and cumbersome assembly is then placed between the press platens and energized with radio frequency energy to seal the components together. Thereafter the entire assembly is removed and the plastic product is detached following which other plastic components are assembled to the fully detached die and inserted in the press for the next sealing operation.
Attempts have been made to increase the size of the platens to permit multiple items to be sealed simultaneously in side-by-side relation as disclosed for example, in the U.S. Pat. No. 4,043,858 to Dantowitz. However, problems are encountered unless the individual items are relatively small. This is because of difficulties encountered in obtaining uniform distribution of electrical energy and sealing effectiveness in larger sized platens. In consequence, a satisfactory seal may be obtained in one area but an unsatisfactory seal in another area which of course results in a commercially unacceptable product. The uneven heating can also cause malfunctioning of the equipment and sticking of the plastic to the die components which results in costly loss of production time and may require removal of the dies for servicing and refinishing. In larger die assemblies it is also much more difficult and costly to achieve and maintain critically essential precise parallelism between the die and the cooperating platen.